1924

For a while, the carbon amplifier patented by Siemens played a major role in hearing aid technology and significantly raised the volume of hearing aids.

The electrical energy controlled by the carbon microphone was not fed to the receiver directly. It first drove the diaphragm of an electromagnetic system connected to a carbon-granule chamber. Current was transmitted across this chamber from the vibrating diaphragm electrode to the fixed electrode plate.

The amplified current produced mechanical vibrations in the electromagnetic hearing diaphragm that were then transmitted to the ear as sound.